NEED INFO.

REBOOT2704

Recruit
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Sep 16, 2022
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1
HELLO I HAVE A 1969 20HP JOHNSON SEAHORSE JUST WONDERING CAN I BEEF UP TO 25HP? MODEL #20R-69D. AND IF SO WHAT CARB WOULD I NEED. AND WHAT OTHERS WOULD I NEED. ANY HELP WOULD BE GREAT. THANKS.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
50,980
welcome aboard

no need for all caps

look at the 20hp parts listing and the 25hp parts listing and see what changes
 

racerone

Supreme Mariner
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Dec 28, 2013
Messages
38,802
This topic and question will not go away. ---Takes more than a carburetor change.-----There were some internal changes. to make more power.----And I believe crankshaft was balanced to allow it to rev at 6000 RPM.-----The 18 / 20 HP was rated at 4500 RPM.-----The 25 was rated at 5500 RPM.
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,226
18-20-25hp motors had different combinations of pistons, connecting rods, crankshaft, and cylinder heads as well as different carbs. And no, you can't just slap a 25hp head on an 18 or 20. It's all or nothing. OMC saw you coming on this one and made sure it wasn't an easy do.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
13,857
Reeds had some differences too.
This is one case/example where the are no differences in the Block/Ports, but that doesn't mean the Port Timing is the same between the engines
 
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